1. The Field of the Invention
The present invention generally relates to medical devices and methods of use, particularly to pedicle screw systems and methods for their installation.
2. The Relevant Technology
Over the past several decades, spinal surgery has increasingly become an important option available to surgeons and patients in treating issues related to the spine. Because the spine generally provides support and movement for the body, a problem with the spine (e.g., a back disorder) can disrupt even the simplest life activities. In general, thousands of surgical fusions of the spine are performed each year in an attempt to decrease pain and to increase function for the patient. Stabilization of the spine through fusion may be accomplished in a variety of ways, including, for example, the use of pedicle screws, e.g., often used to assist in the fixation of the lumbar and thoracic portions of the spine. Such pedicle screws are generally biocompatible screws that are typically anchored into a vertebra at the pedicle, which is a projection from the body of a given vertebra that connects the body of the vertebra to an arch of the vertebra. Vertebrae generally have two pedicles.
Placement of pedicle screws allows the surgeon to minimize or limit the motion of the spine during the healing process, so as to achieve the desired fixation. Placement of pedicle screws is generally considered a relatively safe procedure, although the procedure is not without some inherent risks. For example, such risks include, but are not limited to, screw fracture and/or loosening, increased incidence of infection, and nerve irritation or injury from unintentional interaction between the pedicle screw and the intimately associated nerve root. For example, each pedicle is closely related to its associated nerve root which passes medial and inferior to the pedicle before it exits the spinal canal through the neuroforamin. Due to the proximity of the nerve root to the pedicle, the nerve is at risk for irritation or injury in the event that the pedicle screw breaches the medial or inferior wall of the pedicle.
Numerous studies have demonstrated this risk, with the incidence reported from anywhere between 2 and 20 percent. Encroachments of not more than within 2 millimeters are less likely to be problematic. In an attempt to identify pedicle screws which have breached the wall of the pedicle, and are therefore more likely to cause nerve root irritation, neuromonitoring equipment has been employed. For example, after insertion of the pedicle screw, the screw may be stimulated with an electrical probe. Electrical current enters through the screw head, which is touched by the stimulating probe, and then disperses into the patient through the pedicle screw. Pedicle screws are generally manufactured from metal, with titanium being the most common. Discharge of electrical energy is intended to follow the path of least resistance, e.g., discharging preferentially through the portion of the pedicle screw which has breached the wall of the pedicle.
While such methods can be helpful in determining whether particular placement of the pedicle screw is more likely to result in nerve pain, there exists a continuing need for improved pedicle screws and methods to address these issues.